I have found a list of Plural-forms for many languages, which might get useful for you.

Here in Battle for Wesnoth those lines of plural-forms in the po-files seem correct for the languages we currently have. If your language is still not there, please contact the maintainer of this list to update with your plural definion.

The last value is the localization code you want to use (it_IT for Italian and so on).
The new pot file not only will have the right nplurals value setted on the pot header info, but also will have the right number of msgstr[X] where plural form is used.

For example, on Italian language (like english) you will have only msgstr[0] and msgstr[1] (becouse italian has only two kinds of singular/plural form... [0] in that case is used for singular and [1] for plural).

But other languages could have only ONE form (no plural form at all) and they will have only msgstr[0].
Or they can have more than two forms (for example three)... And in that case, every string with plural form will have msgstr[0], msgstr[1], msgstr[2].... and so on.

It is not suggested, instead, to edit those values by hand.

NOTE: under windows, msginit can be obtained downloading PoEdit, wich contains all the gettext tools .exe under a subdirectory where PoEdit was installed.